In a letter to Obama on Monday, Texas Gov. Greg Abbott (R) called on the Obama administration to halt the resettlement of US refugees altogether to protect national security.

“I urge you, as president, to halt your plans to allow Syrians to be resettled anywhere in the United States. Neither you nor any federal official can guarantee that Syrian refugees will not be part of any terroristic activity. As such, opening our door to them irresponsibly exposes our fellow Americans to unacceptable peril,” Abbott said in the letter.

Experts question how much power governors have in dictating immigration and refugee policy. But the increased pressure has put the Obama administration on the defensive about the program, which planned to allow up to 10,000 refugees from the war-torn country to resettle in fiscal-year 2016.

One major argument immigration advocates have cited against making any changes to current plans is the lengthy refugee-vetting system. It typically lasts 18 to 24 months, and experts say it may actually be among the most difficult ways for terrorists to attempt to enter into the US legally.

“It is extremely unlikely that someone who is a terrorist will be sent through the refugee resettlement program,” Greg Chen, director of advocacy at the American Immigration Lawyers Association, told Business Insider on Monday.

“It takes a great deal of time, and it wouldn’t make sense for someone who is a terrorist for someone to go through that process. There are going to be easier ways for a terrorist to try to infiltrate, rather than going through the refugee resettlement program.”

Senior Obama administration officials laid out how the screening process works in a conference call with reporters on Tuesday.

Multiple intelligence agencies and security departments are involved.

A senior administration official said the lead agency in charge of screenings is the US Citizenship and Immigration Services (USCIS), which is nestled under the Department of Homeland Security.

After DHS agents collect background and biometric information, that information is typically vetted and run through different databases maintained by several agencies, including the State Department, the FBI, and occasionally other intelligence agencies.

The screening process examines and fact-checks available data.

Refugees first register with the United Nations High Commissioner for Refugees. They also provide the US Refugee Admissions Program with their background information — including what caused them to flee and their experience in their home country.

If the refugee who faces imminent persecution in their home country meets one of the five legal qualifications — threat of violence based on race, religion, faith or national origin, political beliefs, or as a member of a targeted social group — then they could potentially qualify for refugee status.

As part of the background check, assessors fact-check the refugee’s biography to ensure that their story is consistent with reports of what has happened on the ground. If a refugee says they fled a bombing in a specific place at a specific time, for example, researchers will work to verify it.

Multiple US departments, including the Department of Defence, also run biometric tests to see if the refugee has been convicted of a crime or has tried to apply for a visa or refugee status previously, among other criteria.

According to one senior administration official, Iraqis and Syrians are more heavily documented than other refugee populations, making it slightly easier to establish basic facts in background checks.

Every refugee applicant gets an in-person interview.

Refugees often have to wait months or even years. But according to one senior administration official, Department of Homeland Security staffers who conduct interviews are given special training from the FBI and some US intelligence agencies for how to handle Iraqi and Syrian refugee cases. Most of the interviews currently take place in Jordan, Turkey, and Egypt.

The US pays extra attention to refugees coming from Syria.

One senior administration official said Syrian refugees go through additional forms of security screening called the “Syrian Enhanced Review,” though the official did not detail what is in the review.

The official also said that some individuals are plucked out and examined by additional US intelligence agencies if their answers raise any red flags.

Of the refugees who have come to the US from Syria so far, half are children, 25% are over the age of 60, and 2% are “combat age” males without who emigrate without family members, a senior official said.

The process hasn’t been flawless.

Senior administration officials told reporters that they have racked up experience by processing so many Iraqi refugee applications.

But they have also run into challenges in the past. As The Washington Post has reported, FBI Director James Comey noted last month that several refugees during the Iraq war who were convicted of terrorism charges managed to make it through the vetting process unnoticed.

“If we don’t know much about somebody, there won’t be anything in our data,” Comey said, according to The Post. “I can’t sit here and offer anybody an absolute assurance that there’s no risk associated with this.”